Investigation of core data reliability to support time‐lapse interpretation in Campos Basin, Brazil

Abstract

In order to calibrate time-lapse quantitative interpretation, it is crucial to analyze saturation and pressure effects on seismic velocities. While the former is adequately modeled using Gassmann equations, the latter is mainly obtained by laboratory measurements, which can be affected by core damage. We investigate the magnitude of this effect on compressional wave velocities by comparing laboratory experiments and log measurements. We use Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. The analysis is performed for an unusual densely cored well from which 43 cores were extracted over a 45 meters thick turbidite reservoir. Comparison of these computed velocities with the sonic log measurements shows very good agreement. This confirms that for this particular region the effect of core damage on M. Grochau and B. Gurevich 2 ultrasonic measurements is below the measurement error. Consequently stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data in this area.

Estimating azimuthal stress‐induced P‐wave anisotropy from S‐wave anisotropy using sonic log or vertical seismic profile data

“…Figures c, c, and c show an approximation of the Poisson's ratio in the hypothetical dry unstressed isotropic rock calculated from equation after deducing the dry P‐wave modulus c 33 from Gassmann equations. The obtained values lie between 0.05 and 0.25, which is consistent with typical values calculated from core measurements on dry isotropic sandstone samples (Mann and Fatt ; King ; Han ; Grochau and Gurevich ). Results obtained for the anisotropy parameters are presented in Figs.…”

Applications of tomography in oil-gas industry – Part 1

“…We have established some understanding of the complex relation between the seismic properties of reservoir and related rocks, and their petrophysical properties in relation to production (porosity, permeability) and state (mineralogy, saturation, pore pressure). The unlocking of the interconnections between seismic attributes and petrophysical parameters is of key importance for the evaluation of stratigraphic traps, fracture detection and the spatial distribution of porosity and permeability (Grochau, 2009;Grochau & Gurevich, 2008;Nur, 1987;Scott, Zaman, & Roegiers, 1998;Shapiro & Gurevich, 2002). Some applications of seismic velocity (and attenuation) imaging in the description of reservoirs and the monitoring of reservoir recovery (Nur, 1987, p. 235) have been demonstrated over the years.…”